The Collaborative International Dictionary of English v.0.48:

Obsolete \Ob"so*lete\, a. [L. obsoletus, p. p. of obsolescere.
   See Obsolescent.]
   1. No longer in use; gone into disuse; disused; neglected;
      as, an obsolete word; an obsolete statute; -- applied
      chiefly to words, writings, or observances.
      [1913 Webster]

   2. (Biol.) Not very distinct; obscure; rudimental;
      imperfectly developed; abortive.
      [1913 Webster]

   Syn: Ancient; antiquated; old-fashioned; antique; old;
        disused; neglected. See Ancient.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Obsolete \Ob"so*lete\, v. i.
   To become obsolete; to go out of use. [R.] --Fitzed. Hall.
   [1913 Webster]

The Devil's Dictionary (1881-1906):

OBSOLETE, adj.  No longer used by the timid.  Said chiefly of words.
A word which some lexicographer has marked obsolete is ever thereafter
an object of dread and loathing to the fool writer, but if it is a
good word and has no exact modern equivalent equally good, it is good
enough for the good writer.  Indeed, a writer's attitude toward
"obsolete" words is as true a measure of his literary ability as
anything except the character of his work.  A dictionary of obsolete
and obsolescent words would not only be singularly rich in strong and
sweet parts of speech; it would add large possessions to the
vocabulary of every competent writer who might not happen to be a
competent reader.


Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "obsolete":
   abandoned, abjured, ago, ancient, antediluvian, antiquate,
   antiquated, antique, archaic, behind the times, blown over, by,
   bygone, bypast, dated, dead, dead and buried, deceased, defunct,
   demode, departed, deserted, discarded, discontinued, disused,
   done with, elapsed, expired, extinct, finished, forgotten, gone,
   gone glimmering, gone out, gone-by, has-been, hors de combat,
   irrecoverable, lapsed, no more, not worth saving, obsolesce,
   obsolescent, off the field, old, old hat, old-fashioned, old-time,
   old-timey, on the shelf, out, out of commission, out of date,
   out of fashion, out of gear, out of style, out of use, out-of-date,
   outdated, outmoded, outworn, over, passe, passed, passed away,
   past, past use, pensioned off, relinquished, renounced, resigned,
   retired, run out, superannuate, superannuated, superseded, unused,
   vanished, worn-out, wound up